Annular eclipses happen when the Moon is at or near apogee, the point in its orbit when its farthest from Earth. It looks a bit smaller because it's farther away, so it doesn't cover the Sun completely. In this case, the eclipse is happening near perihelion, when the Earth is closest to the Sun, so we're maximizing the effect: the Moon looks small and the Sun looks big. When this happens, the Moon leaves a ring of sun around it, called an annulus (Latin for ring). Hence annular.